c – Qt：QWidget :: paintEngine：不应再被调用

问题：
在施工时,QPainter抛出此警告：

QWidget::paintEngine: Should no longer be called

然后,抛出其他几个相关的警告：

QPainter::begin: Paint device returned engine == 0, type: 1
QPainter::setPen: Painter not active

难怪：QPainter没有画任何东西……
另外,我应该调用paintEvent();我自己？
或者它应该被QWidget调用每一帧,我不知怎的搞砸了？

我在网上搜索过,但我找到的所有帖子都没有回复,或者他们在哪里
使用QWidget以外的东西.

我的代码：

mfcanvas.cpp：

#include "mfcanvas.h"
#include <QDebug>
#include <QPainter>
#include <QVector2D>
#include <QList>

MFCanvas::MFCanvas(QWidget *parent) : QWidget(parent)
{
    paths = new QList<QList<QVector2D>*>();
    current = NULL;
    QWidget::resize(100, 100);
}

MFCanvas::~MFCanvas()
{
    delete paths;
}

void MFCanvas::paintEvent(QPaintEvent *)
{
    if(current!=NULL){
        if(current->length() > 1){
            QPainter painter(this);
            painter.setPen(Qt::black);
            for(int i = 1; i < current->length(); i++){
                painter.drawLine(current->at(i-1).x(), current->at(i-1).y(), current->at(i).x(), current->at(i).y());
            }
        }
    }
}

void MFCanvas::pressed(float x, float y)
{
    if(current==NULL){
        qDebug() << "null:"<<current;
        current = new QList<QVector2D>();
        current->append(QVector2D(x, y));
    }else{
        qDebug() << "current:"<<current;
    }
    paintEvent(NULL);
}

void MFCanvas::update(float x, float y)
{
    current->append(QVector2D(x, y));
}

void MFCanvas::resize(int w, int h)
{
    QWidget::resize(w, h);
}

main.cpp中：

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QtQml>
#include <QSurfaceFormat>
#include "creator.h"
#include "mfcanvas.h"

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    qmlRegisterType<MFCanvas>("com.cpp.mfcanvas", 1, 0, "MFCanvas");

    QQmlApplicationEngine engine;

    QQmlComponent *component = new QQmlComponent(&engine);
    QObject::connect(&engine, SIGNAL(quit()), QCoreApplication::instance(), SLOT(quit()));

    Creator creator(component);
    QObject::connect(component, SIGNAL(statusChanged(QQmlComponent::Status)), &creator, SLOT(create(QQmlComponent::Status)));

    component->loadUrl(QUrl("qrc:///main.qml"));

    int rv;

    rv = app.exec();
    delete component;
    return rv;
}

creator.cpp：

#include "creator.h"
#include <QQuickWindow>
#include <QDebug>

Creator::Creator(QQmlComponent *component)
{
    this->component = component;
}

void Creator::create(QQmlComponent::Status status)
{
    if(status == QQmlComponent::Ready){
        QObject *topLevel = component->create();
        QQuickWindow::setDefaultAlphaBuffer(true);
        QQuickWindow *window = qobject_cast<QQuickWindow *>(topLevel);

        QSurfaceFormat surfaceFormat = window->requestedFormat();
        window->setFormat(surfaceFormat);
        window->show();
    }
}

main.qml :(重要部分)

import QtQuick 2.2
import QtQuick.Controls 1.1
import QtQuick.Controls.Styles 1.2
import QtQuick.Layouts 1.1
import QtQuick.Window 2.0
import com.cpp.mfcanvas 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("MFCanvas")

    onSceneGraphInitialized: {
        drawMenu.visible = true;
        lineWidth.visible = true;
        colorMenu.visible = true;
        drawMenu.visible = false;
        lineWidth.visible = false;
        colorMenu.visible = false;
    }

    Rectangle {
        id: main
        anchors.fill: parent

        property real toolsH: 15
        property real iconW: 25

        property real menuH: 8
        property real menuW: 16

        property real dpi: (Screen.logicalPixelDensity == undefined ? 6 : Screen.logicalPixelDensity) * 1.5

        property color choosenColor: Qt.hsla(hue.value, saturation.value, luminance.value, 1)

        Text {
            anchors.centerIn: parent
            font.pointSize: 60
            text: "MFCanvas"
        }

        MFCanvas {
            id: canvas
            Component.onCompleted: {
                canvas.resize(main.width, main.height);
            }
        }
    //...
    }
}

如果您需要任何其他信息,请告诉我.
先感谢您！ =)

https://forum.qt.io/topic/64693

简而言之：不要试图直接从输入事件处理程序绘制,
但是重载窗口小部件中的paintEvent方法并创建
QPain在那里.仅使用输入事件来修改内部
数据模型并使用paintEvent中的QPainter在输出路径上显示它.